Optical diffusers are devices that scatter light. Optical diffusers can be used in conjunction with optical fiber to introduce scattered light into specific locations that are otherwise hard to reach.
Optical excitation of biological systems is an important application of this concept, and includes Photo-Dynamic Therapy (PDT) and opto-genetics. Many devices have been developed to accomplish this simple sounding task. These devices consist of an optical source, a delivery device, such as an optical fiber, and a device for coupling light out of the delivery fiber and into the biological system. The coupling device or diffuser is often complicated, expensive to produce, mechanically fragile, and cannot be easily redesigned for different applications.
In the case of PDT, optical excitation is typically in the visible red portion of the electromagnetic spectrum, requires Continuous Wave (CW) excitation, narrow bandwidth and high power (W). For PDT applications, it is desirable that the diffuser illuminates the surrounding region in a uniform, or Lambertian distribution. An optical diffuser suitable for PDT in one context may well be inappropriate for different applications.
An example of an FDA approved PDT drug is Photofrin™ or porfimer sodium for use against esophageal cancer and endobronchial cancer. The porfimer sodium is injected intravenously, and 40-50 hours after injection the area is illuminated with laser light. According to the product monograph, the laser system must be approved to deliver a stable power output at a wavelength of 630 plus/minus 3 nm. Light is delivered to the tumor by fiber optic diffusers passed through the operating channel of an endoscope/bronchoscope.
An optical diffuser designed to work with a specific light source and fiber optic cable to reliably deliver a 630 plus/minus 3 nm (i.e. to work with porfimer sodium) may be less effective or even ineffective when used with other PDT drugs.
Conventional optical diffusers use coreless optical fibers and/or bulk scattered elements.
For example, the use of coreless optical fiber for use as an optical termination or “beam dump” is described in U.S. Pat. No. 5,263,103 of Kosinski, “Low Reflection Optical Fiber Termination”. In U.S. Pat. No. 5,263,103 a coreless optical fiber is fabricated in the standard way, with the single exception that the core is omitted. The index of the glass is chosen to match, or nearly match that of the core of the fiber it is to be spliced or otherwise mated to. The surface of the resulting splice or mating region can be recoated to produce a seamless splice between the two fiber types. Optically, the coreless fiber behaves as a lossy waveguide, which is the reason for its desirability as a termination device. The coreless fiber is used as a “beam dump”, i.e. a fiber where light entering the coreless fiber produces essentially no reflected energy. A typical application would be as a dump for residual infra-red pump light in an Erbium doped optical amplifier.
Coreless fiber has also been used as end-caps for cored fiber, to allow high power beams to focus through the coreless fiber without exposing the core. The incoming beam presents a larger spot on the exposed coreless face, and hence a lower energy density, compared to focusing directly on an exposed core, thus avoiding optically induced damage.
For use as a diffuser, the loss per unit length of coreless fiber is low, so that for a high level of diffusion a length of many cm—typically greater than 10 cm—is required. In contrast, diffusers for use in PDT should be relatively short (as they are inserted into biological environments), typically 5 cm or less. To achieve desired scattering in a length of 5 cm or less, a coreless fiber would need to be modified to increase surface scattering, either by roughening the surface, by coating the surface with a material containing scattering particles, or by coating the surface with a plastic heat shrink tubing that scatters light, or by cutting threads into the glass.
Scattering can also be achieved through bulk scattering elements. For example, U.S. Pat. No. 5,074,632 discloses creating a cavity at the end of optical fiber and filling the cavity with a scattering material. U.S. Pat. No. 5,946,441 discloses coupling light out at a nose come attached to optical fiber, and also discloses abrading the surface of the nose-cone to increase scattering. Note that in doing so, the nose cone can be of greater diameter than the optic fiber, which is undesirable when used for PDT or other applications requiring insertion into biological systems.
Diffraction via gratings is disclosed in U.S. Pat. No. 6,398,778 in which Bragg gratings are formed in the distal end of the fiber to couple light out of the fiber. The gratings are typically formed using a planar mask, so the resulting grating couples light in a preferred direction. Multiple gratings can be fabricated to reduce the directionality but results in increased cost and reduced mechanical strength.
The use of chemical etching is disclosed in U.S. Pat. No. 6,004,315. To increase scattering from a diffuser made from conventional, glass-cored fiber, by the process disclosed in U.S. Pat. No. 6,004,315, the cladding must be removed. Cladding removal uses a preferential chemical etch process and requires careful implementation and results in a significantly weakened fiber.
In summary the prior art processes have various drawbacks: in some cases complications and expense in construction, in other cases inefficient scattering resulting in long lengths needed to achieve a desired scattering, and in other cases the diffuser is fragile, easy to break and difficult to maintain.